An electrostatic fluid jet applicator is designed to apply a fluid (e.g., a liquid dye) to a moving substrate (e.g., a fabric) by selectively charging and recovering some of the fluid droplets continuously ejected from a stationary linear array of orifices affixed transverse to the movement of the substrate, while allowing uncharged droplets to strike the substrate (e.g., thereby forming an image on the substrate).
In the prior art, electrostatic fluid jet applicators having pattern generating capability typically include an array of charge electrodes (e.g., one for each orifice jet in the array). In addition, the pattern to be printed is typically stored in an electronic digital memory in the form of picture elements (which are referred to as pixels). Lines of image data must be read out from the memory to an array of individual charge voltage control circuits to apply the appropriate control voltage to each individual charge electrode as determined by the image data. See U.S. Pat. No. 3,956,756, which is an example of an electrostatic fluid jet applicator having pattern generating capability.
Such pattern generating electrostatic fluid jet applicators are typically extremely expensive and may well possess more extensive pattern generating capabilities than some users need or desire.
The fluid jet applicator of the present invention can be controlled to uniformly apply solid shades to a fabric substrate, and to produce many different patterns. Yet, the applicator requires no digital memory device to store extensive image data defining each pixel of the patterns to be printed. Moreover, the applicator does not require individual pixel data to be fed to each charge electrode control circuit. Rather, the applicator of the present invention, may utilize simply a single elongated charging electrode which is utilized to simultaneously charge (or not charge) droplets emanating from each of the jets in the orifice array.
With regard to operating in the solid shade mode (and in regard to generating patterns having substantial solid shade portions), there is a need to uniformly apply fluid. The applicator of the present invention includes control circuitry for insuring uniformity when such is required.
It is contemplated for the present invention to be employed generally in the textile and other industries. The applicator of the present invention provides a solution to many of the formidable problems associated with, for example, applying fluid to an entire range of commercial fabrics.
As explained in the commonly-assigned copending U.S. Pat. No. 4,523,202 to Gamblin, if "ink" (actually many suitable liquid treatments may be used) jet electrostatic printing techniques are to be employed generally in the textile industry, random droplet formation processes may be utilized--as opposed to more conventional use of regular periodically stimulated droplet formation processes.
Even if conventional regular periodic stimulation is employed, there may still be a degree of randomness involved (especially when very short print times are considered).
In brief, the need for random droplet formation processes arises from the fact that typical textile applications may require cross-machine orifice arrays considerably in excess of the approximately only 8-10 inches cross-machine dimension typically utilized for printing onto paper of standard letter and legal sizes where regular periodically stimulated non-random droplet formation processes are purposely employed. When cross-machine dimensions much larger than 8-10 inches are required (e.g., perhaps up to approximately 1.8 meters in many typical textile applications), such regular periodic acoustic stimulation of the liquid so as to produce a non-random droplet formation process inevitably generates standing acoustic waves (or other adverse phenomena) within the applicator and/or liquid so as to generate undesirable variations in printing quality along the cross-machine dimension. For example "cusps" and/or "nulls" in the quantity of delivered liquid may form along the elongated cross-machine orifice array. To avoid such standing waves or other adverse phenomena (and thus permit longer cross-machine dimensions for single orifice arrays), Gamblin has proposed the purposeful employment of random droplet formation processes. As explained more fully in the above-referenced application, Gamblin proposes either (a) utilizing no stimulation at all (but even this probably inherently utilizes naturally occurring random acoustic vibrations or other ambient random processes to stimulate random droplet formation as described by Lord Rayleigh over a century ago) or (b) purposefully generating non-periodic (i.e., noise or pseudo-random) stimulations in the fluid jets issuing from orifices along a linear array of such orifices and thus causing a random droplet formation process to occur along the array. Since there are no coherent sources of regular periodic acoustic energy within the system, the maintenance of standing acoustic waves is necessarily avoided (i.e., because there are no regular travelling waves moving in opposite directions so as to constructively add and subtract thus forming cusps and nulls in a standing pressure wave pattern) nor are other such adverse phenomena permitted to exist. Typically, random or pseudo-random electrical signals are generated and fed to an electroacoustic transducer which is acoustically coupled to the liquid jets as they stream outward from the orifices.
In other words, there are situations in which it is either desirable or necessary to utilize random droplet formation processes within a liquid jet electrostatic applicator. The random drop formation processes may be entirely natural (i.e., totally without any artificial drop formation stimulation) or with use of a randomized artificial stimulation process. In this context, a single linear array of liquid jet orifices is typically employed to randomly generate a corresponding linear array of downwardly falling droplets formed at random time intervals and having a random distribution of droplet sizes. During a given "print time" interval, the droplets then passing by a charging electrode zone will not be charged and thus they will continue falling downward to impact with a substrate (e.g., a textile fabric) positioned therebelow (i.e., so as to be dyed, printed or otherwise treated by the liquid). Between such "print time" intervals, are located spacing time intervals during which the droplets are charged and subsequently deflected downstream in a further electrostatic field toward a droplet catching structure.
One of the reasons that liquid jet electrostatic applicators were thought to have potential advantage in the textile industry is that it was hoped that one might achieve a fairly tight control over the amount of fluid that is actually applied to the textile in a given treating process (e.g., dyeing). In many conventional textile liquid treatment processes, a considerable amount of excess "add-on" liquid is necessarily applied to the textile. Subsequently, much effort and expense are typically encountered in removing this excess fluid from the textile. For example, some of the excess might be physically squeezed out of the textile (e.g., by passage through opposed rollers) but much of it will have to be evaporated by heated air flows or the like. This requires considerable investment of equipment, energy, time and real estate. In addition, there is an obvious loss of the sometimes precious treating material itself--unless it is somehow recaptured and recycled which in itself involves yet further additional expense, effort, etc.
Accordingly, if one can somehow apply only the needed amount of liquid "add-on" treatment to a fabric, there is considerable economic advantage to be had.
At the same time, in many applications (e.g., textile dyeing operations), the treating liquid must be uniformly distributed throughout the treated substrate if one is to achieve a commercially acceptable product. Furthermore, in typical commercial environments, it will be necessary for a single apparatus to successfully treat a wide variety of different types of textile substrates each having different requirements if one is to achieve uniformity.
For example, for solid shade dyeing in textile applications, the liquid jet applicator must be able to apply fluid in a uniform fashion to an entire range of commercial fabrics. Different styles of fabric vary considerably in terms of fiber content, yarn size, construction, weave and preparation. These general parameters, when combined, in turn determine relative physical properties and characteristics of a given fabric such as porosity, weight, wetability, capillary diffusion (wicking) and the like. As will be appreciated, the volume of fluid per unit surface area required to adequately treat a given fabric is greatly influenced by these physical properties.
In order to control the volume of liquid per unit area passing onto the substrate moving therepast in a liquid jet electrostatic applicator, it was initially thought that one would merely have to control the duty cycle or "print time" of a fixed repetitive total cycle time interval (assuming a constant substrate velocity). That is, if a given print time is assumed to deposit a "packet" of droplets to form a corresponding printed "pixel" (i.e., a "picture element") on the substrate, and if the center-to-center pixel spacing is fixed at some predetermined small increment (e.g., 0.010 inch or 0.016 inch), then it was initially assumed that one merely had to control the volume of liquid deposited in each such closely-spaced pixel area to control the overall volume of applied liquid per unit area.
However, when actual laboratory experiments were run and applied "add-on" fluid volumes were thus controlled, it was found necessary to reduce the print time to durations of relative small magnitudes (e.g., on the order of 50-100 microseconds). In this manner, it was expected that only relatively small "packets" of droplets (hence small volumes of liquid) would impinge upon each of relatively closely-spaced center points in the textile medium such that the expected droplet spread diameter (typically wicking on the order of ten times the drop diameter can be expected in a fabric) would ultimately result in a uniform distribution of dyestuff within the textile medium.
Surprisingly, this straightforward approach did not produce uniform liquid applications. Instead, attempts to use this early approach revealed severe non-uniformity in the delivered liquid volumes along the linear orifice array. Further experiment and subsequent statistical analysis have revealed that the standard deviation of delivered liquid volumes along the linear orifice array increases exponentially as the print time interval is decreased. This result was evident not only in measured volumes of elements across the linear orifice array but also in the visual and optically measured appearance of dyed or printed textile substrates. It was discovered, for example, that when print time intervals on the order of 75-100 microseconds were employed (for center-to-center pixel spacings of 0.016 inch), volume variations in delivered liquid along the linear array are on the order of .+-.25%. Once this problem became apparent, it appeared to present a possibly insurmountable obstacle in the path of a desired uniform dye shade liquid jet electrostatic applicator machine using random droplet formation processes.
However, further consideration has led to a better understanding of the phenomena underlying this problem of apparent non-uniformity when print times are reduced significantly to controllably limit the average liquid volume per unit area being applied to the fabric. For example, although the term "random droplet formation processes" necessarily implies lack of regular or periodic droplet formation, nevertheless, a statistical average or mean droplet formation rate in such systems is predetermined by system parameters such as the liquid (e.g., its viscosity), the liquid pressure acting on the orifices, and the orifice diameter. For systems thought to be of interest in the textile industry, the mean or average random droplet formation rate is typically in the range of 20,000 to 50,000 drops per second (i.e., one drop every 20 to 50 microseconds). Once that fact is in hand, it can be seen that the relatively short print times of 50-100 microseconds earlier referenced mean that only a relatively few (e.g., two or three) droplets can, on the average, be expected to constitute the "packet" of droplets selected for printing purposes during such a short print time. Accordingly, random variations in the number of such droplets (e.g., the addition or subtraction of one such droplet) within a given print time interval will result in a considerable variation in the total volume of fluid delivered during a given unit print time interval. The result was the observed non-uniformity of printing volumes released along the linear orifice array at any given time and, therefore, deposited upon the imprinted fabric or other substrate medium.
Once these phenomena were better understood, it was then observed that improved uniformity of delivered liquid volume per unit distance along the orifice array could be obtained only by using print times in excess of approximately 200 microseconds (e.g., where the statistical standard deviation of volume delivered to the substrate is expected to be no more than about 0.2) with continued increases in uniformity being observed as the print time intervals were increased. Unfortunately, however, such increased print time intervals (now known to be necessary to achieve the desired uniformity of delivered liquid volume per unit distance along the linear array orifice) also increased the average overall volume being delivered per unit area of the textile substrate being dyed or printed. Such increases in delivered volume per unit area directly conflict with the desired advantage of providing only the optimum required amount of "add-on" liquid (e.g., low wet pickup dyeing of textiles) so as to avoid subsequent problems caused by the use of excess liquid volumes in the first place.
Even though the center-to-center pixel spacings on the substrate had earlier been selected and fixed for a given fabric at distances where the expected wicking or other diffusion processes would result in uniform distribution of applied liquid between the pixel centers, it was next theorized that since increased delivered volumes were now being supplied in each packet of droplets at a given pixel site, one might be able to move the pixel centers further apart and still maintain uniform final distribution--but now without the use of excess "add-on" liquid volume. That is, it was theorized that the above-stated problems might all be simultaneously overcome if one were to maintain relatively longer minimum print times (so as to average random variations in the number of droplets occurring along the linear array during any given print time) coupled with correspondingly longer elapsed time intervals between such print times (i.e., larger center-to-center pixel spacings). Further restated, the minimum amount of fluid being delivered to each pixel on the textile substrate during each print time was increased but the linear spacing on the substrate between such pixels was simultaneously increased so as to still achieve only the desired optimum overall volume/weight of liquid per unit area being delivered to the textile surface. (As will be appreciated, if the textile substrate is moved at a known given relative velocity in the longitudinal or "machine" direction, then the spacing interval distance on the substrate will also correspond to a known time interval).
Color uniformity of commercial fabric is judged not only across one surface, but also front-to-back, side-to-side and even within the thickness of the fabric. Overall color must be uniform in each of these areas for the product to be commercially acceptable. In normal "pad" dyeing, the pad pressure forces dye (i.e., by direct contact) into the fabric interior from both sides of the cloth. This assures that all areas of the substrate are exposed to the dye and results in uniform color throughout the fabric.
Liquid jet electrostatic application, on the other hand, being a non-contact form of application does not impart any significant mechanical work to the fabric in the dyeing process so as to aid in color distribution on the substrate. Rather, dye or color uniformity is achieved solely by movement of the fluid itself once it is deposited at a given location on the fabric surface. In textile applications, such movement is governed to a large extent by the physical properties and characteristics of the fabric as previously mentioned. These parameters determine how well a dye can move within the fabric microstructure and, thus, the degree to which the dye can become distributed within the fabric. Such parameters can differ drastically among fabrics.
Since fabric characteristics are to a large extent fixed by consumer demands, only the application parameters of the instrument are available for manipulation so as to assure uniform coloring of the fabric, these parameters being, for example, orifice size, print pulse width and pixel spacing. Orifice size and fluid pressure and the like are primarily set by the maximum fluid volume requirements so as to cover a given range of fabrics to be processed by a given machine setup. When operating in the solid shade mode, the desired degree of fluid "add-on" (i.e., the average volume per unit area of fluid delivered to the substrate surface) is controlled by maintaining the print pulse width above a predetermined minimum level while at the same time adjusting the center-to-center pixel spacing as may be required. In this manner, a greater range of fabrics may be satisfactorily treated by a single machine setup of a liquid jet electrostatic applicator utilizing random droplet formation processes.
The area of textile surface dyed or printed due to the impingement of a single packet of randomly formed droplets generated by a single orifice has been observed empirically to increase roughly as the square root of the selected print time. That is, for an increase of print time of 2X, a corresponding increase in the longitudinal or machine direction center-to-center spacing of pixels or print "packets" of droplets upon the substrate of 1.4142X would be required. This relationship is believed to be affected by the physical properties and characteristics of a given textile medium but has been observed to be generally true for light to medium weight (e.g., 1 to 8 ounces per yard) woven fabrics. In the exemplary embodiment, when operating in the solid shade mode, typical values of print times and longitudinal spacing range from 250 microseconds at 0.030 inch center-to-center pixel spacing to 550 microsecond print times at 0.040 inch center-to-center pixel spacing. It should be noted that these values are typical but in no way limit the scope of the invention in that each individual substrate will require its own distinct set of operating parameters.
These as well as other objects and advantages of this invention will be better appreciated by reading the following detailed description of the presently preferred exemplary embodiment taken in conjunction with the accompanying drawings.